Symbolic matlab: how to set a variable as a function of time?

Hello to all! I'm trying to set a symbolic variable as a function of "t". This is because I need to obtain something like this:

K>> diff(sin(theta),t)

ans = diff(theta,t)*cos(theta)

or

ans= theta_dot*cos(theta)

but how to tell Matlab that variable theta is a function of time "t"?

Thank you!

[Ezequiel Garcia]

You need to declare t as a "symbolic"

syms t;

Answer 1

[Walter Roberson]

syms theta t
diff(sin(theta(t)),t)

[Walter Roberson]

In newer versions of MATLAB, you can use

 syms theta(t)
 diff(sin(theta),t)

[Sergei Sergienko]

Is it possible to make vector function of t? Something like this

sym('q(t)', [3 1])
diff(sin(q),t)

Answer 2

[sharara rehimi]

thanks a lot..great for my problem

Answer 3

[Wayne King]

Hi Sebastian, do you know the expression for theta(t)?

syms theta t;
theta(t) = t^2;
diff(cos(theta),t)

Answer 4

[Sebastian D'Amico]

Wayne, thank you for your answer! I don't have the expression of theta because I don't need it. This is the reason I need to tell Matlab that theta is a function of "t" so, when derivate is computed, it should consider that, for example, diff(sin(theta),t) is equal to diff(theta,t)*cos(theta). But, how to set it?

Seabstian

[Wayne King]

Do you really need that if you do not know the expression for theta(t), you know df(theta)/dt = df/dtheta*dtheta/dt